The invention relates to a vertical boat for holding a plurality of semiconductor wafers and a method for making the same, which comprises two end members and a plurality of support members vertically mounted on the end members.
In an oxidation and/or a diffusion process, a wafer boat supporting a plurality of semiconductor wafers is transferred into a diffusion furnace, and the wafers are subjected to a predetermined heat treatment.
Either a vertical boat or a horizontal boat is selected according to the type of the diffusion furnace.
A conventional vertical boat has two end plates and a plurality of, for example four, vertical support bar members fixed between and to the end plates. A plurality of wafers are held in a horizontal or somewhat inclined manner on the vertical boat. A plurality of slits are formed on each vertical support member at a predetermined interval for receiving the semiconductor wafers.
In the conventional vertical boat, all vertical support members usually have a uniform configuration in cross section. The vertical support member is, for instance, a triangle bar or a rectangle bar. Each slit formed on each vertical support member has an arc-shaped inner bottom corresponding to the periphery of the wafer. Thus, a support surface formed on each slit has only a small extension in the circumferential direction of the wafer.
On the other hand, Japanese Utility Model laid open No. 62-128633 discloses a boat which comprises circular arc plates fixed on support bars, wherein the loading stress of the large wafer can be somewhat relaxed. However this boat is expensive because the circular arc plates must have a very accurate surface on which the wafer is to be placed.
During heat treatments not only wafers but also the boat, regardless of its shape, is heated to a high temperature. The larger the wafer is, the more it is apt to be deflected by its own weight when heated to such a high temperature. It is because the wafer is supported only at its peripheral portion by the conventional vertical boat. Thus the wafer is apt to be deformed and sometimes slipped, and a crystal dislocation therein is caused. Consequently the yield will go down. The term "slip" is used in reference to a crystal defect or a crystal dislocation.
On the other hand, the load of the wafers is not equally distributed among the vertical support members. In other words the load is concentrated to a specific support member or members. As a result, the larger the wafer is, the more frequently slips take place.
In the above-stated conventional boat, two front vertical support members located on the wafer inserting side are spaced somewhat larger than the outer diameter of the wafer for forming a space between the two front vertical support members large enough to insert and to take out the wafers. When the wafers are properly loaded on the vertical support members, the angle between a line linking a front edge of the front vertical support members to the gravity center of the wafers and the wafer inserting direction is about 90 degrees. In such a position, the gravity center of the wafer is approximately located between the front edges of the two front vertical support members so that the load of the wafers is biased on the two front vertical support members. As a result the two front vertical support members share, for example, 70-90% of the weight of the wafers.
There is also a thermal stress resulting from a temperature difference between the wafers and the slit portions of the vertical support member engaging the wafers.
Therefore the larger the wafers are, more frequently slip dislocations are caused.
When the wafers are subjected to a heat treatment, heat is transferred to the wafers not only by radiation from the diffusion furnace but also by direct heat transfer from the slit portions of the vertical support members engaging with the wafers. The latter, particularly, results in a partial temperature difference on each wafer. Stress caused on the wafer not only by its own weight but also by the temperature difference leads to slip dislocation of the wafers especially near the two front vertical support members. As a result, during heat treatments, the yield of wafers will go down.